The present invention relates to an oxygen sensor for detecting the concentration of oxygen contained in a subject gas, and more particularly to an oxygen sensor used to control an air-fuel ratio of a mixture air in an internal combustion engine by detecting the concentration of oxygen in exhaust gas of the engine.
An oxygen sensor of this type is disclosed, for example, in U.S. Pat. No. 4,121,988 and Japanese Patent Disclosure No. 100746/83. The conventional oxygen sensor has a solid electrolytic element made of an oxygen ion conducting metal oxide. The element has a first surface exposed to exhaust gas of an internal combustion engine and a second surface exposed to atmospheric air as a reference gas. First and second electrodes are respectively fixed to the first and second surfaces of the element. An electrically insulating layer made of a heat resistant ceramic material is formed on the first surface of the element except that portion to which the first electrode is fixed. Lead of thin film electrically connected to the first electrode is further formed on the insulating layer. Additionally, a gas diffusion resisting layer made of a porous electrically insulating ceramic material is formed on the first electrode, the insulating layer and the lead.
When a predetermined voltage is applied between the first electrode and the second electrode of this oxygen sensor, a current proportional to the concentration of oxygen in the exhaust gas flows between the first electrode and the second electrode, and the concentration of oxygen in the exhaust gas can be detected by measuring the current.
However, although the lead of the oxygen sensor is covered with a resisting layer, the lead is exposed to the exhaust gas (which largely varies in temperature) through the resisting layer. Further, since the resisting layer covered on the lead is porous and the outer surface of the insulating layer formed on the lead is rough, metallic particles of the lead of the thin film are apt to cohere when the oxygen sensor is used for a long period of time. Thus, it is a drawback that the lead of the oxygen sensor is locally disconnected at some positions. Therefore, the electric resistance of the lead increases. The lead is completely disconnected in the worst case. As a result, it is impossible to detect the concentration of oxygen.